Electrical contact



y 1953 T. J. DUNSHEATH 2,639,355

ELECTRICAL CONTACT Filed May 25, 1951 2 Sheets-Sheet l [Hz EH T02" MJZW,,w' m

May 19, 1953 T. J. DUNSHEATH 2,639,355

- ELECTRICAL CONTACT Filed May 25, 1951 2 Sheets-Sheet 2 5i a d-L 7.: i-L v 711% w A. 57' 9 Jul/EJ72271 1720mm JfiZZ/ZJ/Zamfi 2E wa PatentedMay 19, 1953 ELECTRICAL CONTACT Thomas J. Dunsheath, Chicago, Ill.,assignor to Magnaflux Corporation, Chicago, 111., a corporation ofDelaware Application May '25, 1951, Serial No. 228,318

The present invention relates to electrical contacts, particularly foruse where high current carrying capacity contacts are required.

The contacts of the present invention will find use in a variety offields, but in particular are extremely useful in machines designed todetect flaws in metallic test pieces. In this type of machine, a testsample is inspected for fiaws by supporting the element to be testedbetween a pairof electrical contact elements, applying a thin-layer offerromagnetic particles, preferably in the form of a suspension in oil,over the test piece and then passing a high amperage current through thetest piece. The applied current sets up a magnetic field about the testpiece, and the ferromagnetic particles become clustered at'any surfaceflaws or fissures present in the article. After termination of thecurrent flow, the presence of such flaws or fissures can be determinedin the test piece by a visual observation, as the clusteredferromagnetic particles around a flaw or other defect will be ofsufficient size to be readily notice-j able. It has been conventionalpractice in machines of this nature to place the piece to be tested be,tween a pair of electrical contacting heads consisting of contact padsor plates mounted on air cylinder rods, with the air cylinders beingoperated to clamp the piece. The electrical current is then passedthrough the piece for approxi; mately one-half second by the operationof a re lay in the electrical circuit of the machine. The currentintensities in the test piece present during the application of thecurrent are quite high, being on the order of 1,000 amps. for a testpiece having a diameter of one inch. Because of the extremely highcurrent densities, there is definite danger of burning the test piece atthe point where the contact is made, particularly for parts havingirregular end surfaces. This burning presents a very serious problem,particularly where finished parts are being inspected, as the currentmars the finish of the parts. In addition, excessive heating of the testpiece even for short intervals of time is sometimes sufficient to changethe hardness of the test piece in localized areas. Where the contactelements are made'of copper, a further problem is presented, because thecop per mayenter the grain structure of the steel or other metal beingtested, and cause copperembrittlement of the piece being tested.

In addition to other requirements, the contact elements must besufiiciently rugged to have an extended life. Many inspecting machinesof the type described operate at rates as high as 1,800

7 Claims. (Cl. 200166) test pieces per hour, and the replacement ofcontacts in such machines is undesirable from the standpoint of delaysnecessitated by shutting down the equipment to replace the contacts,

In general, machines of the nature described have employed either leadplates or copper braid pads with a backing member of synthetic rubber ascontact elements. Lead plates have the advantage that they do notordinarily burn the test piece, since the lead melts and conforms to thesurface of the piece being tested. However, lead plates have a shortuseful life, and are objectionable because there is a tendency tosplatter molten lead away from the contacts, and endanger the safety ofthe operator.

I While copper braid contact pads have a longer life than lead plates,they also have the disadvantage of having a greater tendency to burn thepiece being tested. In addition, as the copper braids wear out, finecopper wires broken from the braid are deposited in the tank used todrain the suspension of ferromagnetic particles and cause clogging.

To overcome the disadvantages of the lead plates and copper braid padswhich have previously been employed as contacts in machines of thisnature, three requirements must be met. First, the amount of heatgenerated at the point of contact between the contact element and testpiece must be reduced. Since the current cannot be reduced without areduction in the effectiveness of the inspecting process, the contactresistance must necessarily be lowered. Secondly, the heat generated atthe contact area must be spread over a large area, or distributed overseveral points rather than at a localized small area to prevent intenseconcentration of the heat evolved. Third, the contact elementsthemselves should have suificient thermal capacity and conductivity todraw heat away rapidly from the point of contact.

In order to obtain low contact resistance between the electricalcontacts and the test piece, several requirements should be met. In thefirst instance, the contact material itself should have a low electricalresistance. Also, the contact pressure between the test piece and thecontact elements should be high. If possible, the contact shouldincorporate a wiping action. In addition, the metal from which thecontact is made should not form a non-conducting oxide.

Forthe efficient removal of heat from the areas of contact, the contactelements should mass. Finally, the design of the contact units sent asubstantially flat contacting surface 33 along the entire length of thecontact element 25.

The leaves 35 are prevented from falling out of .the enclosure byproviding a slightly offcenter aperture 38 in each of the leaves 35.;Upon placing the leaves in the interleaving relation ship shown inFigure 2, the apertures 38 are in registry with each other, to receive arelatively 6 ting contact with the supports 55 and 56, with resultingslight deformation of the resilient members 51 and 58. Y

The leaves 54' are prevented from falling out of the enclosure 50 bymeans of a pair of retaining strips 60 and 6|, the retaining stripsbeing held in position by means of a pair of bolts 62 small diameterbolt 39 therethrough. The bolt 39 is threaded at both ends and rigidlysecured to the end walls of the enclosure 26 by means of a pair of nutsand 4|. The relationship of the rod 39 and the individual leaves 35 isbest illustrated in Figure 5 of the drawings. The rod 39 and the nuts 40and 4| hold the leaves 35 with suflicient pressure to prevent lateralmovement of the leaves 35, but still provide sufi'icient workingclearance to allow independent pivotal movement of the leaves 35 aboutthe support blocks 28 and 29.-

The engagement between the'contact assembly and a test piece isillustrated in Figures 4 and 5. As shown in these drawings, as a testpiece 42 is engaged between a pair of opposed contact elements 25, thecontacting leaf elements are individually deflected. For example, asshown in Figure 4, the leaf 35a is pivoted about the end portionssupported on the support block 28 so that its free end 351) compressesand deforms the resilient member 3|.- In the same way, the leaf 35cpivots about the point of contact between one end of the leaf 35c andthe contact block 29, so that its free end 35d compresses the oppositeresilient member 30. The number of leaves 35 Which are engaged by agiven work piece will, of course, depend upon the width of theindividual leaves 35 and the dimensions of the test piece 42. However,as best seen in Figures 4 and 5, the test piece 42, is engaged by-theleaves 35 in a series of spaced contact points, generally indicated atnumerals 44, 45, 46- and 41 of Figure 5 so that the heat evolved uponapplication of current from the energizing means (not shown) in thehousings l9 and 20 will be more effectively dissipated. As the testpiece 42 is removed, the natural resiliency of the resilient members 30and 3| urges the individual leaves 35 back into their initial alignment,determined by engagement of the apertures 38 with the rod 39.

In the embodiment of; the invention illustrated in Figures 6 and '7, themeans for retaining the leaves within the enclosure are different fromthat illustrated in previous figures. As shown in Figure 6, thismodified form of the invention consists of a contact element comprisinga box-like enclosure 50 composed of brass or other electricallyconductive material, with a pair of integral extensions 5| extendingfrom opposite sides thereof. Each of the extensions 5| is provided withan aperture 52 for receiving a bolt or other securing means to enablethe contact element to be secured within the housing of the testingmachine.

As in the case of the previously described embodiment, the contactelement of Figures 6- and 7 contains a plurality of elongated conductingleaves or blades 54 disposed in interleaving relationship between theend walls of the box-like enclosure 50.

As illustrated in Figure 7, the contact structure also includes a pairof oppositely disposed cylindrical supports 55 and 55, and a pair ofsubstantially cylindrical resilient members 51 and 58 in abuttingcontact therewith, A spacer 59 urges the resilient members 5! and 58into abutand 63 and their associated nuts 54 and 66.

Instead of providing each of the leaves 54 with a slightly off-centeraperture as in the case of the previously described embodiment, theleaves 64 of the contact of Figures 6 and '7 are provided with apertures65 at an end portion of the individual leaves. When the leaves arealigned in interleaving relationship, that is, with an end portion ofone leaf 54 being supported on the cylindrical support member 56, andits other end support on the resilient member 5'|,*and.the

next adjoining leaf 54 having its apertured end supported by the supportmember '55 at its free end by the resilient member 58, the apertures 65are in registry to receive the securing bolts 62 and 63 therethrough, asillustrated in Figure 7. This construction has the advantage that theindividual leaves are stronger, since the apertures are not punched fromthe center of the individual leaves, but in an end portion. However,this embodiment is not as useful as the previouslyment of the testpieces61 with the leaves 54 is such that a plurality of contact points isestablished between the end surface of the test piece 61 and thecontacting surfaces of the individual leaves 54. Upon removal of thetest piece 61, the natural resiliency of the resilient members 51 and 58is sufficient to urge the ends of the depressed leaves 54 outwardly sothat they present a flat contacting surface.

From the foregoing, it will be appreciated that the contact of thepresent invention has severalv distinct advantages over previously usedhigh current carrying electrical contacts. The individually deflectibleleaf elements of the contact assembly provide a plurality of contactpoints when engaged by a test piece, to minimize the danger of excessiveheat concentration, while still holding the test piece rigidly. It willalso be observed that as a test piece is clamped between the testingheads, a slight wiping action occurs as the individual leaves aredeflected into a V- shape. This wiping action is important inestablishing the low contact resistance between the contact element andthe surface of the test piece.

In actual trials, the contact elements of the present invention havebeen 'shown to be far superior to previously employed contact elementsusing copper braid contacts. In test runs involving the testing ofautomobile connecting rods, currents of 3000 amperes could be passedinto the test part without burning of the piece. This current value isabout twice that possible when using copper braided contact elements.Fur-thermore, the contacts of the present invention have a longer usefullife than either the lead plate or 7 copper braid type of contactspreviously used in testing machines of this nature.

It will be understood that modifications and variations may be effectedwithout departing from the scope of the novel concepts of the presentinvention.

I claim as my invention:

1. An electrical contact comprising an electrically conductive support,a pair of electrically conductive contact elements in spaced parallelrelation along said support, a plurality of elongated conductingelements bridging the space between said contact elements, saidconducting elements being arranged with an end portion of one otsaidconducting elements supported on one of'said contact elements, and anend portion of the next adjacent conducting element being supported onthe other of said contact elements.

-2. An electrical contact comprising a support, span of electricallyconductive contact elements in spaced parallel relationship along saidsupport, a pair of resiliently deformable members in spaced parallelrelation along said support, a plurality of elongated conductingelements, said conducting elements being arranged with an end 7 portionof one of said conducting elements supported on one of said contactelements, and the other end of said conducting elements being supportedon one of said resiliently deformable members, and the next adjacentconducting element having one end portion supported on the other of saidcontact elements, and its other end portion supported on the other ofsaid resiliently deformable members.

3. An electrical contact comprising an electrlcally conductive support,a pair of electrically conductive contact elements disposed in spacedparallel relationship along the support, a resil iently deformablemember associatedwith each of said contact elements, a spacer urgingeach of said resiliently deformable members into abutting contact withits associated contact element, and a plurality of elongated conductingelements in interleaving engagement, said conducting elements beingsupported along said support by said contact elements and saidresiliently deformable members.

1. An electrical contact comprising an electrically conductive support,a pair of spaced contact elements along said support, a plurality ofelongated electrically conductive members disposed along said support ininterleaving relationship, with portions of said conductive elementsbeing supportedby said contact elements, means associated with saidsupport defining a pivotal axis transverse to said elongatedelectrically conductive members, and resilient means supporting 8portions "of said-electrically conductive members; said interleavingelectrically conductive members defining-a relatively flat surface, saidspaced contact defining a pivotal axis for each of said'electricallyconductive members.

5. An electrical contact comprising an electri-v cally conductivesupport, a pair of spaced contact elements disposed in parallelrelationship along said support, a' plurality of elongated aperturedelectrically conductive members disposed along said support interleavingrelationship with portions ofsaid conductive members bein supported bysaid :contact elements; resilient means supporting other portions'ofsaid electrically conductive members, said'interleaving'electricallyconductive members defining a relatively flat surface, and a rod securedto said support and extending through the apertures of saidelectricallyconductive members.

6. An electrical contact comprising an electr'l cally conductivesupport, a pair of parallel spaced contact elements in spaced alignmentalong said support, a plurality of elongated ele'ctricallyconductivemembers, each of said electrically conductive' members having aperturedend portions supported on and in electrical contact with one of saidcontact elements, said electrically conductiveelements being arrangedinterleaving relationship along said contact elements, and a pair ofrods secure-d to said support and extending through the apertures insaid electrically conductive members.

7. An electrical contact assembly comprising electrically conductiveelements in interleaved arrangement with the corresponding ends ofalternate elements extending into substantial alignment beyond the endsof adjacentv elements, a rigid conductivemember supporting each set ofextended ends to leave the other ends unsupported, means yiel'dinglysupporting said other ends to bring the working surfaces of saidelements into substantially the same plane, and means having a lostmotion connection with all of said elements providing limitedindependent movement or each of said yieldably supported ends when awork piece is brought into pressure contact-with a portion of saidworking surfaces intermediate said rigid members.

'lZI-IOMAS J. DUNSHEATH.

, References Cited in the file or this patent UNITED STATES PATENTSNumber Name Date 1,978,246 Bauerschmidt Oct. 23, 1934 2,268,161

